1. Field the Invention
The present invention relates to a gas sensor for measuring oxygen concentration, air-fuel ratio state, NOx concentration, etc., in exhaust gas of an internal combustion engine.
2. Description of the Related Art
A gas sensor employing a detection element made of a solid electrolyte such as zirconia is known as a gas sensor for detecting oxygen concentration, etc., in a gas to be measured such as exhaust gas. A bottomed cylinder-shaped (cup-shaped or test tube-shaped) element having a closed front end, a rod-shaped (plate-shaped) element made of a laminate of sheets, etc., can be used as the detection element. Any of these detection elements (hereinafter also referred to as an “element”) has a detection portion provided at its front end and which is brought into contact with the gas to be measured. The element is formed so that the detection portion is exposed to the gas to be measured. For example, in an oxygen sensor having a bottomed cylinder-shaped detection element made of an oxygen ion conductor, the detection element is positioned and fixed into a cylindrical metal shell such that the front end side of the element protrudes from a front end side of the metal shell. The metal shell (hereinafter also referred to as “shell”) is attached to an exhaust pipe of an internal combustion engine in such manner that the shell is screwed into the exhaust pipe, for example, by a male thread formed on the outer circumferential-surface of the shell. The shell is arranged such that the front end of the detection element protrudes into the exhaust pipe. Thus, while an electrode layer (measurement electrode layer) formed on the outer circumferential surface (outer surface) of the detection element contacts the exhaust gas, an electrode layer (reference electrode layer) formed on the inner circumferential surface (inner surface) of the detection element contacts a reference oxygen-containing gas (e.g., atmospheric air) to thereby generate an electromotive force (electric potential difference) between the two electrodes in accordance with the oxygen concentration difference between the inner and outer circumferential surfaces of the element. A signal based on the electromotive force is output to a control circuit so that the oxygen concentration is detected and the air-field ratio is controlled.
This type gas sensor is attached to the exhaust gas pipe so that the front end side of the element protrudes into the exhaust gas pipe. Upon attachment, there is a possibility that the element will crack because of collision of the front end side of the element with the exhaust gas pipe. Because the front end of the element is exposed to a severe atmosphere or condition such as high-temperature exhaust gas or a heat cycle, it is necessary to protect the detection element of a solid electrolyte or the electrode layer formed on the front surface of the detection element. Therefore, a metal element protection cap (also called “protection pipe”, “protector” or “element cover”) provided with an adequate number of ventilation through-holes (ventholes) is generally attached to the front end side of the element. As a result, the exhaust gas can pass through the ventholes so that the exhaust gas contacts the measurement electrode layer on the outer circumferential surface of the element.
This type element protection cap (hereinafter also referred to as a “cap”) is shaped like a cup, etc., so that the front end side of the element protruding from the front end side of the metal shell is covered with the cap. Typical means for fixing the element protection cap to the metal shell include a welding structure in which laser welding or resistance welding is applied after the cap is fitted to the front end side of the shell, or a fixing means which uses crimping (Japanese Patent Publication No. 15221/1993). The crimping is performed as follows. First, a protrusion portion such as a flange protruding to the outside (outwards) is provided on the outer circumference of an open end in the rear end side of the element protection cap. On the other hand, a front-end-facing end surface is provided in a front end region of the metal shell and a crimping cylindrical portion is provided in the outer circumference of the front-end-facing end surface so that an end region including the flange can be enclosed in the crimping cylindrical portion when the element protection cap is inserted in the metal shell. In a condition such that the end region is disposed in the cylindrical portion, the cylindrical portion is compressively deformed in an axial direction while being bent inward. The compressive deformation of the cylindrical portion, that is, crimping, allows the flange to be intensively pressed against the front-end-facing end surface side of the metal shell to thereby fix the element protection cap to the metal shell. In the fixing structure which uses crimping, a technique for interposing a metal ring (packing) in order to improve the fixing force is known (Japanese Utility Model Laid-Open No. 186061/1987).
3. Problems to be Solved by the Invention
In the fixing structure of the element protection cap (protection pipe) described in Japanese Patent Publication No. 15221/1993, a problem of rattling or moving may occur in the element protection cap. This is because a turn-back edge of the compressively deformed crimping portion is loosened by high-temperature exhaust gas or a repeated heat cycle so that the element protection cap is completely fixed. Particularly in this structure, reliability of the fixing force is problematic because the element protection cap is apt to be loosened around the axis of the male thread of the shell. It is considered that the cause of loosening in the fixing structure using crimping is the presence of residual stress generated in respective parts at the time of crimping, the buckling of the compressively deformed portion, etc., in addition to the heat cycle.
In the fixing structure of the element protection cap (protector) described in Japanese Utility Model Laid-Open No. 186061/1987, an increase in fixing force is attained by interposing a the metal ring packing, but there is still a risk that the crimping portion may be loosened.